The invention relates to a semiconductor device comprising a semiconductor body provided with a radiation-sensitive element comprising a transistor having an emitter region and a collector region of a first conductivity type and an interposed base region of a second, opposite conductivity type and comprising a radiation-sensitive region having a rectifying junction, which is conductively connected to the base region of the transistor.
From the handbook: "Physics of Semiconductor Devices, Second Ed.; p. 784" of S. M. Sze, such a semiconductor device is known, in which the radiation-sensitive element comprises a phototransistor. The base region of the transistor is constituted by a p-type semiconductor zone, which is provided in an n-type silicon substrate. The substrate itself serves as collector region of the transistor. An n-type emitter region of the transistor is located in the base region. During operation a depletion region is situated around the pn junction between the base region and the collector region, which constitutes the photosensitive region of the device.
A semiconductor device of the kind mentioned above is used to convert radiation into an electrical signal. The signal produced can then be further processed electronically. Especially in recent years the possibilities of use of such devices have increased, such as, for example, in the optical telecommunication and in the optical or magnetooptical recording and reading of information on an information carrier, designated briefly hereinafter as optical recording, such as, for example, DOR (Direct Optical Recording), CD (Compact Disk), VLP and computer applications, such as CDROM/CDI.
The use of radiation-sensitive diodes for reading a CD is described in Philips Technical Review, Vol. 40, 1982, No. 6, pp. 148 ff. There, a system of four adjacent photodiodes is used for reading the CD. The information is stored on the CD in tracks, which are scanned by a laser beam. In the reflected beam, the information is represented by intensity variations, which are detected by the diodes and are converted into an electrical signal.
A radiation-sensitive element having a radiation-sensitive transistor instead of a diode has the great advantage that a considerably larger output signal can be obtained. In a photodiode, the output signal is limited to the photocurrent produced, whereas in a transistor with a suitable amplification factor the output signal can be many times larger. However, a radiation-sensitive transistor generally has a considerably lower (detection) speed than a radiation-sensitive diode having an equally large radiation-sensitive surface. Due to the parasitic capacitance of the radiation-sensitive junction of the transistor, the transistor can no longer follow adequately the optical signal at higher frequencies. This renders such a transistor unsuitable for high-frequency applications, such as optical recording. For example, with CD, the information carrier is read out at a speed of more than 1 m/s, which means that the reflected beam can vary in intensity well over a million times per second.